This paper presents a diagnostic approach to study the spatial and temporal variability of total water storage (TWS) properties of hydrologic basins. The approach utilizes a simple water balance equation based on the physical law of mass conservation. Thirty-six years of monthly historical records of precipitation and streamflow discharge data were collected from 27 basins in the Arkansas-Red River basin. Five different assumptions were made to estimate evapotranspiration. The study found that the assumption that considers both initial total water storage and precipitation amount in the current period provides the most consistent long-term seasonal evapotranspiration, when compared to seasonal evapotranspiration derived from the atmospheric water budget in the Arkansas-Red River basin. The TWS variability range has an apparent east-west gradient and is highly correlated to climatological factors such as the ratio of annual precipitation and potential evapotranspiration and vegetation information. Wetter basins tend to have a larger TWS variability range than dry ones. The intra-annual TWS variability range is small compared to year-to-year variability. This phenomenon is more noticeable for dry basins, implying that longer data sets are needed to study the water balance of dry regions. Another finding is that the TWS variability range is shown to be scale dependent, with intermediate-scale-area basins exhibiting larger variability than large-scale-area basin.